This invention relates to elimination of spurious signals in double conversion tuners by dynamic selection of the frequency of a first IF signal and selection of a crystal frequency. Such operation reduces or eliminates interference caused by spurious signals generated by the combination of first and second local oscillator frequencies and their harmonics, as well as by changes to the crystal reference frequency.
In double conversion tuners, two mixers are used to convert an incoming signal into a predetermined intermediate frequency (IF) output signal called the IF output. Each mixer must use a local oscillator (LO) to tune a desired channel. Each LO produces a local oscillator signal of a certain frequency. The frequency of each LO signal is controlled by a frequency synthesizer circuit which in turn is controlled by means of digital words provided by an external controller. The first mixer receives a group of incoming signals that are combined with the first-LO signal to select one of the incoming channels and convert it to a first IF signal. This first IF signal is then passed through a second mixer that receives a second LO signal from a second LO to convert the first IF signal into a second IF signal. The second IF signal is filtered and amplified to produce the IF output.
Conventionally, double conversion tuners use a first IF signal with a fixed frequency for the entire range of input channels. The frequency of this first IF signal is determined in different ways. For example, in order to avoid image frequencies, it is frequently convenient to set the first IF signal to as high a frequency as possible. Currently, there are on the market many double conversion tuners and it can be said that almost every manufacturer has selected a first IF signal frequency that is considered to be optimum or proper to a particular tuner design because of cost, topology, and circuit limitations.
A double conversion tuner has many sections, any of them can generate spurious signals that cause interference with tuner operation if not properly isolated or filtered. Either (or both) of the local oscillators (LO) can produce spurious signals, when an LO signal or its harmonics are combined with other signals. Some of the spurious signals can be generated by the crystal oscillator. External signals can also generate spurious signals in a tuner when they combine with preexisting spurious signals. Generally any or all of the above-described spurious signals are undesirable and, if possible, should be reduced or eliminated to enhance tuner operation.
The following equations describe the two types of spurious signals which cause most of the interference with tuner operation.
Equation (1) describes the spurious signals generated by the local oscillators:
Spurious Signal=ABS((first LO frequency*n)-(second LO frequency*n))xe2x80x83xe2x80x83(1)
Where:
n=1,2,3,4,5,6,7, . . . .
ABS=Mathematical Absolute Value *=Multiplier symbol
Equations (2) and (3) describe two kinds of interference generated by the crystal reference frequency:
Spurious Signal=IF outputxe2x88x92Xtlxe2x80x83xe2x80x83(2)
Spurious Signal=IF outputxe2x88x92(Fch1xe2x88x92Fch2xe2x88x92Xtl)xe2x80x83xe2x80x83(3)
Where, for equation (2) and equation (3):
IF output=second intermediate frequency (IF) signal.
Fch1,2=video frequency of the channel.
Xtl=crystal reference frequency.
It is a technical advantage of the present invention to provide techniques to reduce or eliminate spurious signals in the bandwidth of the IF output signal of a double conversion tuner. As mentioned before, conventional double conversion tuners have a fixed first IF value. Moreover, the second LO signal frequency in such conventional tuners is the same for all the tunable input channels. If the first IF signal frequency becomes dynamic then it is possible to select a specific first IF frequency per channel in such way as to eliminate the possible combinations between the harmonics of the first LO signal and the second LO signal that can cause an interference in the bandwidth of the IF output signal. The invention is embodied in this dynamic IF technique. For example, according to the invention, on an NTSC system, the bandwidth of the IF output signal includes those frequencies whose values are greater than or equal to a second IF signal frequency xe2x88x924.75 MHz and are less than or equal to a second IF signal frequency +1.25 MHz; other system standards have different bandwidths.
Most of the double conversion tuners use a 4 Mhz crystal reference frequency. If the crystal reference frequency modulates the IF output signal then spurious signals might appear in the bandwidth of the IF output signal. The invention is further embodied in a technique to move spurious signals from the bandwidth of the IF output signal by selection of a crystal reference frequency such that, when the crystal reference frequency is tested according to equations (2) and (3), the result is outside the IF output bandwidth.
Hereinafter, the following definitions are used:
xe2x80x9cIF output bandwidthxe2x80x9d - Bandwidth of the IF output signal;
xe2x80x9cfirst IFxe2x80x9d - The IF signal produced by the first mixer;
xe2x80x9csecond IFxe2x80x9d - The signal produced by the second mixer;
xe2x80x9cfirst LO signalxe2x80x9d - The signal produced by the first LO;
xe2x80x9csecond LO signalxe2x80x9d - The signal produced by the second LO;
xe2x80x9cIF output bandwidthxe2x80x9d - The bandwidth of the IF output signal produced by the tuner.